The trend toward increased downlink capacity requirements in cellular radio communications systems is continuing with the expanding menu of high data rate services. Typical network planning strategies for cellular radio communication networks present drawbacks in regard to user handoff and system load balancing for an interference-limited, high-data-rate environment.
Cellular radio communication systems (hereafter “communication systems”) are typically comprised of a number of cells, with each cell corresponding roughly to a geographical area. Each cell has an associated base station (BS) which is a local central cite providing access to the communication system to a number of radio transmitter/receiver units (user terminals (UTs)) within the cell. A BS includes at least one antenna and a transceiver system providing radio service within the cell. A base station of a typical communication system may have three antennas, oriented 120 degrees apart, defining three cells (also referred to as sectors[GDC1]). The BSs couple to base station controllers (BSCs) with each BSC serving a plurality of BSs. The BSCs also couple to a mobile switching center (MSC) which may interface other MCSs and the Public Switched Telephone Network (PSTN). Together, the BSs, BSCs and the MSCs form a cellular radio communication system or network[GDC2]. Network planners typically consider many factors in determining network features such as location of the BSs, BS transmission power (e.g., pilot power) as well as BS antenna physical characteristics (e.g., antenna downtilt). The factors considered may include population demographics, geographical formations, and UT use patterns, among others. Typically such network features remain static and cannot be adjusted to suit dynamic network requirements.
In typical radio cellular communications systems the BS communicates with each UT using a separate temporary radio channel. A channel is a set of two connections, the downlink, for transmitting to the UT and the uplink, for receiving from the UT. Due to the limited number of radio channel frequencies available to support such networks, the efficient use and reuse of available channels is critical to increasing network capacity. Frequency reuse is based on the concept of assigning a distinct group of channels to each neighboring cell, and then assigning the same group of channels to cells that are far enough away from each other so that their use of the same frequencies does not result in substantial interference. The number of cells that are assigned distinct channel groups determines the frequency reuse factor.
Some communication systems employ a low frequency reuse factor. For example, code division multiple access (CDMA) systems including cdma2000, wideband CDMA (W-CDMA), and IS95, and high data rate (HDR), as well as enhanced data GSM environment (EDGE), employ a frequency reuse factor of 1. That is, CDMA systems employ a direct sequence spread spectrum technology in which all of the available frequencies are used in each cell. Each UT within a cell is assigned a distinct orthogonal spreading code that uniquely identifies the UT and is used to decode transmissions from the BS. This technology increases the number of UTs that can be served by a BS by permitting all UTs within a cell to transmit simultaneously in the allocated frequency band. However, since each cell is using the entire frequency band, disjoint groups of channel frequencies are not available for use in neighboring cells. Using the entire frequency band can lead to mutual interference by proximate UTs located in different cells as described below.
In a typical communication system, each BS may serve an area comprised of multiple cells with the shape of each cell defined by the antenna pattern of one or more BS antennas. The size of each cell is determined by the pilot signal strength (pilot power) transmitted from the BS. The pilot signal is an unmodulated, direct-sequence spread spectrum signal, transmitted continuously by each BS, for each cell, which allows each UT to acquire the timing of the downlink channel. When a UT becomes operational, it measures the pilot power of the surrounding BSs. The UT establishes communication with the BS having the strongest pilot power.
Mutual Interference Between User Terminals
UTs in a CDMA system located within the same cell typically do not exhibit significant mutual interference due to the fact that each UT is assigned a mutually orthogonal downlink transmission code (code). Therefore, for UTs within a cell, the interference (other than noise and implementation effects) is virtually zero (if the multi-path spread is not too large). For such UTs, high-data-rate downlink transmissions are possible[GDC3]. This is because, in the CDMA system, total chip transmission rate remains constant, therefore, if a higher data rate is required the spreading factor of the codes must be reduced[GDC4]. The spreading factor of the codes is directly related to the amount of interference through which communication signals may be discerned. So, for UTs exhibiting little mutual interference the spreading factor may be reduced substantially thereby allowing a proportionally higher downlink data transmission rate. However, because each cell uses a different scrambling code set, the codes assigned to UTs of different cells are not guaranteed mutual orthogonality. Therefore, a high-data-rate transmission to such UTs may require a spreading factor reduced to such a degree that the communication signal cannot be discerned in the presence of mutual interference. In such case, a UT experiencing interference from a neighboring cell will request more power in order to maintain a signal-to-interference-plus-noise ratio (SINR) required, for example, by the quality of service (QoS) specified. The increased power to one UT increases the interference to the proximate UT of the neighboring cell. Eventually, some UTs get dropped and capacity is reduced. Therefore, proximate UTs located in different cells may not be capable of high-data-rate downlink transmissions due to mutual interference. Reducing this mutual interference between UTs is becoming more imperative as the trend toward greater downlink capacity requirements continues.
Load Balancing
Load balancing can also cause mutual interference in a high downlink data transmission rate environment. The number of simultaneous, active users (capacity) served within a particular cell may be limited by BS throughput or number of available codes. Thus, the number of users that may be supported by each cell is limited. For this reason it is desirable to distribute the number of active UTs as evenly as possible among the available cells (i.e., balance the load). There are a number of ways to effect load balancing. One such method involves adjusting the pilot power of one or more cells to change the size of the cell thereby encompassing more, or less, UTs to be served through a particular cell.
Moreover, because the codes assigned to each UT for uplink transmissions are not orthogonal as they are for the downlink transmissions, a balanced load is effective when the performance of the system is limited by BS throughput or number of UTs that can be served (uplink limited environment). However, if the limiting factor is interference between UTs, then a balanced load may actually exacerbate the problem. This is because inter-cell code orthogonality is not provided, and load balancing may cause several proximate UTs to be served by different cells having non-orthogonal spreading codes, thus increasing mutual interference. Such mutual interference is particularly problematic in a high-data-rate downlink environment. That is, because each of two, proximate, high-data-rate UTs having non-orthogonal codes may produce the effective equivalent mutual interference of a large group of typical UTs having non-orthogonal codes, the likelihood of such mutual interference causing degradation of system performance is increased in a high-data-rate environment.
Soft Handover
Another aspect of typical CDMA cellular radio communications systems that may degrade system performance through intercell interference in a high downlink data rate environment is that of “soft handover”. If a UT is at a cell border, the power is limited and the channel is weak. Soft handover is a way to compensate for the poor quality by serving the UT through two or more cells, this corresponds to UTs being in a region where two or more cell boundaries overlap. The UT generates a measurement report that includes the pilot power of each cell through which the UT is being served (active set) as well as the pilot power of the neighboring cells (neighboring set). This information is used to determine when the UT should be placed in, or taken out, of soft handover. The measurement report may include various performance measurements in addition to pilot power (e.g. a pilot power-to-interference ratio). This information is reported to the BSCs. Where the UT is being served through cells provided by BSs that are coupled to different BSCs, the measurement report may be forwarded to the MSC.
Soft handover works well for uplink transmissions, though for downlink transmissions, much of the gain of soft handover is lost. This is because soft handover provides large gains when the UT receives equal power through each of the two cells, but it is difficult to align transmission power so that the UT receives equal power through each of the two cells. Moreover if one of the channels is not fading, then soft handover is not necessary anyway. In the downlink, the interference due to being served through two or more cells using non-orthogonal codes may vitiate the benefits of soft handover.